The invention relates to methods for forming luminescent powder phosphors for use in display devices.
Display devices, such as field emission devices (FEDs), cathode ray tubes (CRTs), electroluminescent (EL) displays, LCD backlights, plasma discharge displays, and vacuum fluorescent displays (VFDs) all utilize phosphor materials. For example, CRT displays, such as those commonly used in desk-top computer screens, create an image when a scanning electron beam from an electron gun impinges on phosphors disposed on a relatively distant screen, thereby increasing the energy content of the phosphors. This energy is converted and released as luminescence (light emission) through the phosphor-bearing screen.
Similarly, FEDs (which operate at anode voltages well below cathode ray tube displays) combine the cathodoluminescent phosphor technology of CRT displays with integrated circuit technology to create thin, high-resolution displays wherein each pixel is activated by its own electron emitter or collection of emitters. More specifically, cold cathode emitters arranged in a matrix-addressable array emit electrons that excite phosphors on a screen.
While phosphors can convert energy from impinging electrons into luminescence, phosphors may be excited by other types of energy, such as x-rays, gamma rays, various forms of radiation (e.g., ultraviolet, visible or infrared) and electric fields.
Typically, phosphors have at least two ingredients: the first and largest portion is the "host" or "matrix"; and the second, which is provided in amounts ranging from a few parts per million to several percent, is the "activator." As is well known in the art, a variety of powder materials may serve as the host, including some silicates, phosphates, sulfides and oxides of calcium, yttrium, gadolinium, lanthanum, and zinc. Similarly, a number of known powder materials may function as activators, including manganese, copper, silver, thullium, cerium, antimony and some of the rare-earth elements. Phosphor preparation, including the incorporation of activators, is typically done through a high-temperature reaction of well-mixed, finely ground powders of the phosphor components, referred to generally as "precursors."
Other approaches have also been used to make phosphor material. In one process, described in U.S. Pat. No. 4,440,831, phosphor material is made by using silicic acid particles and zinc oxide to make zinc silicate, which is combined with manganese to produce phosphor particles having a size typically in a range of approximately three to twenty microns in diameter. While such relatively large particles have some benefits, they are difficult to process for small displays like FEDs, which can have a pixel size of 30 microns to a side. Consequently, particles that are in this size range pose problems. Moreover, this process, which appears to be specific to zinc silicate, is undesirable for FED applications because zinc silicate has rapid "coulombic aging", i.e., it breaks down quickly.
In other processes with other powder precursors, such as those described in U.S. Pat. No. 4,948,527, conventional annealing processes subject the precursors to temperatures ranging from about 1000.degree. C. to 1650.degree. C. over many hours. Such prolonged heating typically leads to a substantial increase in phosphor particle size, e.g., in a range from 7-25 microns in diameter. This annealing operation contributes most significantly to the growth of the phosphor particle size during fabrication of the phosphor.
Because high resolution display screens, such as certain CRTs, FEDs, plasma displays, EL displays, and VFDs, generally require extremely small particle size (i.e., less than or equal to 1 micron), large phosphor particles are typically milled to a smaller size before being placed on a display screen. Such operations, although necessary to obtain the benefits of smaller particles, are detrimental to the light producing efficiency of the subject phosphor and destroy the physical integrity of the particles.
Accordingly, it would be desirable to produce relatively small phosphor particles that are sufficiently small for flat panel display devices without milling operations.